The overall objective of this proposal is to investigate the nature of different signaling pathways involved in the etiopathogenesis of neurofibrillary degeneration of abnormally hyperphosphorylated tau, a hallmark brain lesion of Alzheimer disease (AD), Down syndrome, frontotemporal dementia, and other tauopathies, and employ this information to identify and diagnose the different subgroups of Alzheimer's disease. We postulate that more than one disease mechanism and signaling pathway are involved in producing AD pathology, and that various subgroups of this disease can be identified based on CSF levels of proteins associated with plaques and neurofibrillary tangles and of taus abnormally phosphorylated at various specific sites. To test this hypothesis we propose (1) to develop and validate ultrasensitive bienzyme-recycle ELISAs for various abnormal phosphorylation sites of tau. (2) To determine CSF levels of A[unreadable], ubiquitin and total tau, and tau phosphorylated at various specific sites using the assays developed in Aim #1 in AD and control cases, and identify subgroups of AD based on these data by cluster analysis. APOE genotype frequencies and clinical profiles of each cluster, including symptoms such as depression, hallucinations, hypokinesia, and rigidity, will be analyzed. The % sensitivity and % specificity of each phosphorylation site at appropriate cut-off points will be determined to evaluate its diagnostic potential. (3) To study the relationship of levels of soluble and aggregated A[unreadable]1, 2, ubiquitin and various phosphotaus between CSF and brain in Alzheimer's disease. Levels of soluble and aggregated A[unreadable]2, ubiquitin and various phosphotaus will be assayed by ELISA and radioimmuno-dot-blots in the frozen autopsied brains of AD cases from which lumbar CSFs are available. The levels of these markers in the brain will be correlated to the histopathological staging of the disease, and to the CSF levels of these markers. These studies will help (i) identify subgroups of AD based on CSF markers, (ii) provide a lead on the nature of signaling pathways involved in various subgroups, (iii) reveal the diagnostic potential of CSF levels of tau phosphorylated at different specific sites and (iv) identify the relationship of the CSF levels of A[unreadable], ubiquitin and tau to these markers in the brain and to the various histopathological stages of Alzheimer's disease. Better classification of AD at the molecular level and identification of biomarkers that represent the underlying disease process of various subtypes of the disease, in the long term, will lead to improved diagnosis and better defined treatment opportunities for AD and other tauopathies.